I'm new at all this, but I'm thinking that 90 degrees at 3 weeks old may be too hot. Forgive me if I'm wrong....

I just found this from this site:

The brooder can be heated by using a light bulb with a reflector, available at any hardware store. A 100-watt bulb is usually fine, though some people use an actual heat lamp. The temperature should be 90-100 degrees for the first week or so, then can be reduced by 5 degrees each week thereafter, until the chicks have their feathers (5-8 weeks old). A thermometer in the brooder is helpful, but you can tell if the temperature is right by how the chicks behave. If they are panting and/or huddling in corners farthest from the light, they are too hot. If they huddle together in a ball under the light, they are too cold. You can adjust the distance of the light (or change the wattage of the bulb) until it's right.

If they have somewhere to get out from under the bulb it isn't a problem. You could lower the temp but so long as they aren't having to cram against the sides of the brooder to get out of the light and heat they are fine. I don't know about the twitching.